Electro-luminescent polymers, their preparation and uses

ABSTRACT

An organic polymer comprising a conjugated backbone for transporting negative charge carriers and having a first band gap and at least one side unit pendent from the backbone for transporting positive charge carriers and having a second band gap, wherein the structure of the organic polymer is selected so that the first and second band gaps are distinct from one another in the polymer.

The present invention relates to an organic polymer and uses thereofsuch as in an optical device, and to a process for preparing such apolymer.

Organic electroluminescent devices are known which employ an organicmaterial for light emission. For example, WO 90/13148 describes such adevice comprising a semi-conductor layer comprising a polymer film whichcomprises at least one conjugated polymer situated between electrodes.The polymer film in this case comprises a poly(para-phenylene vinylene)(PPV) film which is capable of light emission when electrons and holesare injected therein. Other polymer layers capable of transporting holesor transporting electrons to the emissive layer may be incorporated intosuch devices.

For organic semiconductors important characteristics are the bindingenergies, measured with respect to the vacuum level of the electronicenergy levels, particularly the “highest occupied molecular orbital”(HOMO) and “lowest unoccupied molecular orbital” (LUMO) levels. Thesecan be estimated from measurements of photoemission and particularlymeasurements of the electrochemical potentials for oxidation andreduction. It is well understood in the field that such energies areaffected by a number of factors, such as the local environment near aninterface, and the point on the curve (peak) from which the value isdetermined. Accordingly, the use of such values is indicative ratherthan quantitative.

FIG. 1 shows a cross section of typical device for emitting light. FIG.2 shows the energy levels across the device. The anode 1 is a layer oftransparent indium-tin oxide (ITO) with a work function of 4.8 electronvolts. The cathode 2 is a LiAl layer with a work function of 2.4electron volts. Between the electrodes is a light emissive layer 3 ofPPV, having a LUMO energy level 5 at around 2.7 electron volts and aHOMO energy level 6 at around 5.2 electron volts. Holes and electronsthat are injected into the device recombine radiatively in the PPVlayer. An important feature of the device is the hole transport layer 4of polyethylene dioxythiophene (PEDOT). This polymer is disclosed in EP0686662. This provides an intermediate energy level at about 4.8electron volts which helps the holes injected from the ITO to reach theHOMO level in the PPV.

It should be noted here that values stated for energy levels, workfunctions etc. are generally illustrative rather than absolute. Forexample, the work function of ITO can vary widely. The applicants havecarried out Kelvin probe measurements which suggest that 4.8 electronvolts is a reasonable value. However, it is well known that the actualvalue can depend on ITO deposition process and history.

Known device structures also may have an electron transport layersituated between the cathode 2 and the light emissive layer 3. Thisprovides an intermediate energy level which help the electrons injectedfrom the cathode to reach the LUMO level of the material constitutingthe light emissive layer. Suitably, the electron transporting layer hasa LUMO energy level between the LUMO energy levels of the cathode andthe light emissive layer.

One disadvantage associated with multiple layered devices is that wherethe layers are deposited from solution it is difficult to avoid onelayer being disrupted when the next is deposited, and problems can arisewith voids or material trapped between the increased number ofinter-layer boundaries.

Appl.Phys.Lett. 51, 913-915 (1987) is concerned with organic thin-filmelectroluminescence. Devices disclosed in this document consist of ahole-transporting layer of an aromatic diamine and an emissive layer of8-hydroxiquinoiene aluminium. ITO is used as the hole-injectingelectrode and a magnesium-silver alloy as the electron-injectingelectrode.

As is disclosed in Nature, 397, 121-128 (1999) TPD is used as a holetransport layer. However, this molecular material has the disadvantagesassociated with using small molecule layers in a device. Similarly, thisdocument discloses that PBD is known as an electron transport layer.Again, this has the disadvantageous device characteristics associatedwith using small molecule layers as compared with polymer layers inelectroluminescent devices.

The use of polymers in general in light emitting devices, andparticularly as charge transport materials is very attractive. Polymersshow excellent device characteristics. These device characteristicsinclude good efficiency, processability and device lifetime.

Poly(arylamines) are disclosed in U.S. Pat. No. 5,728,801 as usefulcharge transport layers in light-emitting diodes. This document furtherdiscloses that triarylamines are used as charge transport materials,specifically positive charge transport materials, because they areeasily oxidised to the corresponding radical cation. The usefulness ofthe possibility of using these polymers in film form is discussed inthis document.

In view of the above, there still remains a need to simplify thestructure of light emitting devices, thus, simplifying manufacturingprocesses and reducing production costs.

The present invention provides an electroluminescent device comprising:a first charge carrier injecting layer for injecting positive chargecarriers; a second charge carrier injecting layer for injecting negativecharge carriers; and a light emissive layer located between the chargecarrier and injecting layers and comprising a mixture of: a firstcomponent for accepting positive charge carriers from the first chargecarrier injecting layer; a second component for accepting negativecharge carriers from the second charge carrier injecting layer; and athird organic light emissive component for accepting and combiningcharge carriers from the first and second components to generate light.

Two or more of the components of the emissive layer may be provided asfunctional chemical units or moieties of a single molecule. Any furthercomponents of the layer may be provided by one or more further moleculesphysically mixed with the said single molecule. Where a single moleculeprovides more than one component those components could be combined as acopolymer (e.g. in main chain, side chain, block or random form). One ormore of the components could be provided as a pendant group of a polymerchain of another one or more of the components.

According to a first aspect of the present invention there is providedan organic polymer comprising: a conjugated backbone for transportingnegative charge carriers and having a first bandgap; and at least oneside unit pendent from the backbone for transporting positive chargecarriers and having a second bandgap, wherein the structure of theorganic polymer is selected so that the first and second bandgaps aredistinct from one another in the polymer.

The organic polymer provided by the present invention solves theproblems of the prior art by reducing the total number of layers andpolymer components required in a light-emitting device. Instead ofrequiring two separate polymers; one to act as a negative chargetransport material and a second to act as a positive charge transportmaterial, the present polymer is capable as acting as both. This reducesthe number of layers required in a light-emitting device. In the casewhere the components of several different layers having differentfunctions are blended, this reduces the number of components needed inthe blend.

In one preferred aspect of the present invention, the conjugatedbackbone comprises a substituted or unsubstituted fluorene or phenylenegroup.

In a further aspect of the present invention, the conjugated backbonecomprises a carbon or nitrogen bridging group. In the case where theconjugated backbone comprises a substituted or unsubstituted fluorenegroup, the side unit preferably is attached to the conjugated backbonevia a carbon bridging group which is C9 of the fluorene group.

A preferred side unit in accordance with the present invention is a sideunit comprising a substituted or unsubstituted triarylamine group. Asmentioned above, the low oxidation potential of triarylamines makes themparticularly suitable as positive charge transport materials.

In a preferred aspect of the present invention, the organic polymer hasa repeat unit comprising:

wherein one or both of R and R′ comprise the at least one side unit andR and R₁ are the same or different.

In one embodiment of the present invention, the repeat unit comprises:

wherein Ar is a substituted or unsubstituted aromatic or heteroaromaticgroup. Preferably, Ar comprises a substituted or unsubstituted fluorenegroup. More preferably, Ar comprises dioctyl fluorene.

The ability to select Ar is an important feature, particularly in thedesign of electroluminescent devices. The structure of Ar may beselected to improve efficiency of a device by modulating the LUMO levelof the conjugated backbone to help electron injection from the cathodeto the light emissive material.

Suitable Ar groups may comprise a repeat unit as disclosed inInternational patent publication No. WO 00/55927. Specifically,preferred Ar groups may comprise a repeat unit selected from:

X and Y may be the same or different and are substituent groups. V andVI, A, B, C and D may be the same or different and are substituentgroups. It is preferred that one or more of X, Y, A, B, C and D isindependently selected from the group consisting of alkyl, aryl,perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl, alkylaryl andarylalkyl groups. One or more of X, Y, A, B, C and D also may behydrogen. It is preferred that one or more of X, Y, A, B, C and D isindependently an unsubstituted, isobutyl group, an n-alkyl, an n-alkoxyor a trifluoromethyl group because they are suitable for helping toselect the HOMO level and/or for improving solubility of the polymer.

wherein R₃ and R₄ are the same or different and are each independently asubstituent group. Preferably, R₃ or R₄ may be selected from hydrogen,alkyl, aryl, perfluoroalkyl, thioalkyl, cyano, alkoxy, heteroaryl,alkylaryl, or arylalkyl. These groups are preferred for the same reasonsas discussed in relation to X, A, B, C and D above. Preferably, forpractical reasons, R₃ and R₄ are the same. More preferably, they are thesame and are each a phenyl group.

In a preferred aspect of the present invention, one or both of R and R′comprises a substituted or unsubstituted triarylamine unit.

Preferably the substituted or unsubstituted triarylamine unit comprises:

wherein X and Y are substituents which do not substantially affect theoverall function of the polymer and are the same or different. Theapplicants have found preferable ranges of n, m and p to be 1≦n≦6, 1≦m≦6and 1≦p≦6.

Optionally, the side unit in a polymer according to the presentinvention is attached to the backbone via spacer group. In particular,the spacer group may be a C1-C10 alkyl group.

Alternatively, the side unit may be attached directly to the conjugatedbackbone, without the presence of a spacer group. Where the side unitcomprises a substituted or unsubstituted triarylamine unit, it ispreferred that the triarylamine unit is attached directly to theconjugated backbone. Where the conjugated backbone comprises a carbon ornitrogen bridging group, it is preferred that the triarylamine unit isattached directly the bridging group.

In a further preferred aspect according to the present invention, theorganic polymer has a repeat unit comprising:

wherein X, Y, n, m and p are as defined above and X′ and Y′ aresubstituents and are the same or different. Again, the applicants havefound that preferred values for n′, m′ and p′ are 1≦n′≦6, 1≦m′≦6 and1≦p′≦6.

X, Y, X′ and Y′ and n, n′, m, m′, p, and p′ may be selectedadvantageously to modify the HOMO level of the at least one side unit soas to help hole injection from the anode to the emissive material. Forthis purpose, the applicants have found that, preferably, one or more ofX, X′, Y or Y′ independently comprises hydrogen, an alkyl group, analkoxy group, a halide group, a cyano group, a heteroaryl, alkylaryl oran arylalkyl group, particularly a trifluoromethyl group, a thioaikylgroup or a substituted or unsubstituted triarylamine unit. It isparticularly preferred that X and Y are the same and are each hydrogenor an alkyl group and X′ and Y′ are the same and are each hydrogen or analkyl group.

Optionally, the present polymer is soluble. The structure of the sideunit and, in particular X, Y, X′ and Y′ may be selected to confer on thepolymer solubility in a particular solvent system, for example fordepositing the polymer on a substrate. Typical solvents include commonorganic for depositing the polymer on a substrate. Typical solventsinclude common organic solvents, for example, THF, toluene, xylene andorganic ink jet ink formulations.

Without being limited to the above preferred groups for X, X′, Y and Y′,it is preferred that the present organic polvmer has a repeat unitcomprising:

The positions of the substituent X, X′, Y and Y′ groups shown inFormulas III, IV and V above are preferred positions only. It should beunderstood that the substituents may be provided at any suitableposition where the substituent does not substantially affect the overallfunction of the polymer (i.e. that it comprises a conjugated backbonefor transporting negative charge carriers and at least one side unitpendent from the backbone for transporting positive charge carriers).

Broadly speaking, polymers according to the present invention includebranched and linear polymers, homopolymers, copolymers, terpolymers andhigher order polymers. It is envisaged that homopolymers in accordancewith the present invention will be of particular interest with regard totheir use in light emitting devices. In this regard, it should be notedthat a hompolymer (i.e. prepared by polymerisation of a single type ofmonomer) may be defined to have more than one different repeat unit.

Where the polymer according to the present invention is a copolymer orhigher order polymer, suitable comonomers will include those comprisingAr as referred to above in relation to formula II.

Preferably, the degree of polymerisation of a polymer according to thepresent invention is at least 3.

In the first aspect of the present invention, there is provided also afilm or coating comprising a polymer according to this invention.

It is envisaged also that a polymer according to the present inventionmay be used for accepting and combining positive and negative chargecarriers in a device to generate light.

In a second aspect of the present invention, there is provided use of anorganic polymer according to the present invention in an optical devicesuch as a luminescent device, preferably an electroluminescent device.Other devices include photoluminescent devices, photovoltaic devices andwaveguides. Other uses include the use of the polymer in a dyecomposition, in a fibre or in a sensor. Typically, the organic polymeris positioned so that it is capable of acting as a charge transportmaterial either in a charge transport layer or as part of a blendedlight emissive layer.

In a third aspect according to the present invention, there is furtherprovided an optical device comprising a first charge carrier injectinglayer for injecting positive charge carriers, a second charge carrierinjecting layer for injecting negative charge carriers and a lightemissive layer located between the charge carrier injecting layers andcomprising an organic component for accepting and combining positive andnegative charge carriers to generate light and an organic polymer inaccordance with the present invention.

The organic component and organic polymer may be blended together in amixture. The mixture may be homogeneous or phase separated.

Accordingly, the present invention also provides a compositioncomprising a blend or mixture comprising at least one polymer accordingto the present invention. Preferably, the blend comprises at least twoor three different polymers.

Alternatively, the light emissive layer may comprise a layer of theorganic component and one or more layers of an organic polymer inaccordance with the present invention.

Further hole transport layers could be added to the above-describedgeneral device structure so as to provide a series of intermediateenergy steps between the anode and the emissive layer. Suitably, thehole transporting layer has a HOMO energy level between the HOMO energylevels of the anode and the present organic polymer.

It would be known to a person skilled in the art how to make repeat unitmonomers in accordance with the present invention.

According to a fourth aspect of the present invention there is provideda compound having general formula VI:E—repeat unit—E′  VIfor use in a polymerisation reaction for the preparation of a polymer,particularly a polymer according to this invention. Also provided is theuse of the above compound for the preparation of a polymer fortransporting holes and electrons in an optical device. The repeat unitin this compound is as defined above. E and E′ are the same or differentand are reactive groups capable of undergoing chain extension.Typically, E and E′ are the same or different and are selected from thegroup consisting of a reactive halide functional group and a reactiveboron derivative group. Preferably, the reactive halide functional groupis selected from the group consisting of F, Cl, Br and I and the boronderivative group is selected from the group consisting of a boronic acidgroup, a boronic ester group and a borane group.

Several different polymerisation methods are known which may be used tomanufacture polymers in accordance with the present invention.

One suitable method is disclosed in International patent publication No.WO 00/53656, the contents of which are incorporated herein by reference.This describes the process for preparing a conjugated polymer, whichcomprises polymerising in a reaction mixture (a) an aromatic monomerhaving at least two reactive boron derivative groups selected from aboronic acid group, a boronic ester group and a borane group, and anaromatic monomer having at least two reactive halide functional groups;or (b) an aromatic monomer having one reactive halide function group andone reactive boron derivative group selected from a boronic acid group,a boronic ester group and a borane group, wherein the reaction mixturecomprises a catalytic amount of a catalyst (e.g palladium), and anorganic base in an amount sufficient to convert the reactive boronderivative groups into —B(OH)₃ anions.

Polymers according to the present invention which have been produced bythis method are particularly advantageous. This is because reactiontimes are short and residual catalvst (e.g. palladium) levels are low.

Another polymerisation method is disclosed in U.S. Pat. No. 5,777,070.Commonly, this process is known as “Suzuki Polymerisation”. The processinvolves contacting monomers having two reactive groups selected fromboronic acid, C1-C6 boronic acid ester, C1-C6 borane and combinationsthereof with aromatic dihalide functional monomers or monomers havingone reactive boronic acid, boronic acid, boranic acid ester or boringgroup and one reactive halide functional group with each other.

A further polymerisation method is known from “Macromolecules”, 31,1099-1103 (1998). The polymerisation reaction involves nickel-mediatedcoupling of dibromide monomers. This method commonly is known as“Yamamoto Polymerisation”.

The present application now will be described in further detail withreference to the accompanying drawings in which:

FIG. 1 shows a cross section of a typical device for emitting light.

FIG. 2 shows the energy levels across the device of FIG. 1.

FIGS. 3 and 4 show suitable reaction schemes for making a monomersuitable for making a polymer in accordance with the present invention.

One particular monomer suitable for making a polymer in accordance withthe present invention can be synthesised in accordance with the reactionscheme shown in FIG. 3 or FIG. 4. In FIG. 3, step 1 is a Sandmeyerreaction, step 2 is a bromination reaction, and step 3 is a modifiedUllmann reaction. The resultant monomer product is about 85% pure andshows a good NMR spectrum. In FIG. 4, step 1 is a reaction with aniline,and step 2 is a reaction with iodobenzene, followed by the modifiedUllmann reaction to discriminate between iodide and bromide.

EXAMPLE 1 Preparation of DiBrPDAF Phenylated Dianilinefluorene-dibromide

Methanesulfonic acid (3.2 mL, 49.3 mmol) was added dropwise to aniline(15 mL, 164.6 mmol). The solid mixture was heated to 180° C. for onehour then cooled to room temperature overnight. Heating was resumed at180° C. for 30 min then 2,7-dibromofluorenone (5.00 g, 14.79 mmol) wasadded portionwise. The temperature was increased to 185° C. and heatingmaintained for a further 22 hours.

The purple solid was treated with toluene and 10% sodium hydroxide untilpH 14. The aqueous phase was removed and the toluene layer was washedwith saturated sodium chloride, dried over MgSO₄, filtered andevaporated.

¹H NMR (CDCl₃) 7.53 (2H, d, J 8.0), 7.45 (2H, d, J 1.2), 7.43 (2H, dd, J1.6, 8.0), 6.92 (4H, d AB, J 8.4), 6.54 (4H, d AB, J 8.4), 3.60 (4H,NH₂); ¹³C NMR (CDCl₃) 154.3, 145.5, 138.1, 134.6, 130.7, 129.5, 129.1,121.9, 121.6, 115.2, 64.6. GC-MS (m/z, relative intensity %) 506 (M⁺,100).

A flask was charged with 1,10-phenanthroline (0.1983 g, 1.10 mmol),copper(I) chloride (0.1091 g, 1.10 mmol), 40 mL of toluene,dibromodianiline fluorene (5.559 g, 10.98 mmol), iodobenzene (14.8 mL,132.7 mmol), potassium hydroxide finely powdered (3.090 g, 55.07 mmol)and further 40 mL of toluene. The mixture was heated to 160° C. for 26hours. After cooled down to room temperature a further 0.200 g of1,10-phenanthroline and 0.110 g of copper(I) chloride was added andheating was resumed at 160° C. for 5 hours. The reaction mixture wascooled down to room temperature then filtered through celite and thesolvent removed.

¹H NMR (CDCl₃) 7.55 (2H, d, J 8.0), 7.52 (2H, t, J 1.6), 7.45 (2H, dd, J2.0, 8.0), 7.21 (8H, t, J 8.0), 7.06 (8H, d, J 7.6), 6.98 (4H, t, J7.2), 6.98 (4H, d AB, J 9.2), 6.90 (4H, d AB, J 9.2); ¹³C NMR (CDCl₃)153.7, 147.7, 147.0, 138.2, 137.9, 131.0, 129.6, 129.4, 128.9, 124.9,123.3, 123.0, 122.0, 121.7, 64.9.

1. An organic polymer comprising: a conjugated backbone for transportingnegative charge carriers and having a first band gap; and at least oneside unit pendant from the backbone for transporting positive chargecarriers and having a second band gap, wherein the structure of theorganic polymer is selected so that the first and second band gaps aredistinct from one another in the polymer.
 2. An organic polymeraccording to claim 1, wherein the conjugated backbone comprises asubstituted or unsubstituted fluorene or phenylene.
 3. An organicpolymer according to claim 2, wherein the conjugated backbone comprisesa carbon or nitrogen bridging group.
 4. An organic polymer according toclaim 1, wherein the at least one side unit comprises a substituted orunsubstituted triarylamine group.
 5. An organic polymer according toclaim 2, which polymer has a repeat unit comprising:

wherein one or both of R and R¹ comprise the at least one side unit andR and R¹ are the same or different.
 6. An organic polymer according toclaim 5, wherein the repeat unit comprises:

where Ar is a substituted or unsubstituted aromatic or heteromaticgroup.
 7. An organic polymer according to claim 6, wherein Ar comprisesdioctyl fluorene.
 8. An organic polymer according to claim 2, whereinone or both of R and R¹ comprises a substituted or unsubstitutedtriarylamine unit.
 9. An organic polymer according to claim 8, whereinthe substituted or unsubstituted triarylamine unit comprises:

wherein X and Y are substituents and are the same or different and 1≦n≦6and 1≦p≦6.
 10. An organic polymer, according to claim 1, wherein theside unit is attached to the backbone via a C1-C10 alkyl group.
 11. Anorganic polymer according to claim 8, wherein the substitutedtriarylamine unit is attached directly to a bridging group.
 12. Anorganic polymer according to claim 5, wherein R and R¹ are the same. 13.An organic polymer according to claim 5, wherein R and R¹ both comprisea substituted or unsubstituted triarylamine unit.
 14. An organic polymeraccording to claim 9, which polymer has a repeat unit comprising:

wherein X′ and Y′ are substituents and are the same or different and1≦n′≦6, 1≦m′≦6, and 1≦p′≦6.
 15. An organic polymer according to claim14, wherein one or more of X, X′, Y, or Y′ independently compriseshydrogen, an alkyl group, a halide group, a cyano group, atrifluoromethyl group or a substituted or unsubstituted triarylamineunit.
 16. An organic polymer according to claim 14, wherein X and Y arethe same and are each hydrogen or an alkyl group and X′ and Y′ are thesame and are each hydrogen or an alkyl group.
 17. An organic polymeraccording to claim 14, which polymer has a repeat unit comprising:

where one or more of X, X′, Y and Y′ independently comprises hydrogen,an alkyl group, a halide group, a cyano group, a trifluoromethyl groupor a substitiuted or unsbstituted triarylamine unit.
 18. An organicaccording to claim 1 comprising a homopolymer.
 19. An organic polymeraccording to claim 1 for use in an optical device.
 20. An organicpolymer according to claim 19, wherein the optical device comprises anelectroluminescent device.
 21. An organic polymer according to claim 1for use in an electroluminescent device.
 22. An optical devicecomprising: a first charge carrier injecting layer for injectingpositive charge carriers; a second charge carrier injecting layer forinjecting negative charge carriers; a light emissive layer locatedbetween the charge carrier injecting layers and comprising an organiccomponent for accepting and combining positive and negative chargecarriers to generate light; a conjugated backbone for transportingnegative charge carriers and having a first band gap; and at least oneside unit pendant from the backbone for transporting positive chargecarriers and having a second band gap, wherein the structure of theorganic polymer is selected so that the first and second band gaps aredistinct from one another in the polymer.
 23. An optical deviceaccording to claim 22, wherein the organic polymer and the organiccomponent are blended.
 24. An optical device according to claim 22,wherein the light emissive layer comprises an organic component layerand one or more organic polymer layers.